Charging ex-tenants for rent!

I moved out at the end of my lease in September and I got billed for February rent. When I called, they said it was a "computer glitch". This is the second time they have done this. Very unprofessional. They even asked me to fax them the bill. Since I don't have a fax machine in my home, I had to go to a copy shop and pay to have this faxed.